In the manufacture of tissue products, it is generally desireable to provide the final product with as much bulk as possible without compromising other product attributes. However, most tissue machines operating today utilize a process known as "wet-pressing", in which a large amount of water is removed from the newly-formed web by mechanically pressing water out of the web in a pressure nip between a pressure roll and the Yankee dryer surface as the web is transferred from a papermaking felt to the Yankee dryer. This wet-pressing step, while an effective dewatering means, compresses the web and causes a marked reduction in the web thickness and hence bulk.
On the other hand, throughdrying processes have been more recently developed in which web compression is avoided as much as possible in order to preserve and enhance the bulk of the web. These processes provide for supporting the web on a coarse mesh fabric while heated air is passed through the web to remove moisture and dry the web. If a Yankee dryer is used at all in the process, it is for creping the web rather than drying, since the web is already dry when it is transferred to the Yankee surface. Transfer to the Yankee, although requiring compression of the web, does not significantly adversely affect web bulk because the papermaking bonds of the web have already been formed and the web is much more resilient in the dry state.
Although throughdried tissue products exhibit good bulk and softness properties, throughdrying tissue machines are expensive to build and operate. Accordingly there is a need for producing higher quality tissue products by modifying existing, conventional wet-pressing tissue machines.